Recently, resistance variable memory elements, which include Programmable Conductive Random Access Memory (PCRAM) elements and molecular memory elements, have been investigated for suitability as semi-volatile and non-volatile random access memory devices. A typical PCRAM device is disclosed in U.S. Pat. No. 6,348,365 assigned to Micron Technology, Inc., and hereby incorporated by reference. Typical molecular memory devices are described in U.S. Patent Application Publication No. 2002/0163831 to Krieger et al, which is hereby incorporated by reference. In typical PCRAM devices, conductive material, such as silver, is incorporated into a chalcogenide material, which is positioned between two electrodes. The unprogrammed PCRAM device is normally in a high resistance “on” state. A write operation programs the PCRAM device to a lower resistance “off” state via application of a threshold voltage potential between the two electrodes. Molecular memory devices function similarly, but utilize a molecular memory element such as a polymer, rather than a chalcogenide glass.
The programmed lower resistance state can remain intact for an indefinite period, typically ranging from hours to weeks, after the voltage potentials are removed. The PCRAM device can be returned to its higher resistance state by applying a reverse voltage potential of about the same order of magnitude as used to write the element to the lower resistance state. Again, the higher resistance state is maintained in a semi-volatile manner once the voltage potential is removed. In this way, such a device can function as a resistance variable memory element having two resistance states, which can be used as two logic states.
Formation of electrode structures for resistance variable memory cells often requires introducing adhesion and barrier layers between layers of conductive materials. Adhesion layers are useful for preventing two or more layers of conductive material from separating during processing of the memory device, while barrier layers can prevent the unwanted migration of metal ions through the memory cell during operation.
FIG. 1B shows an electrode structure 100, utilizing an adhesion/barrier layer 110 as described above. The introduction of such adhesion/barrier layers 110, however, may result in an upper electrode contact surface 103 being formed of multiple materials, as shown in FIG. 1B. By virtue of being formed of multiple materials, the upper electrode contact surface 103 exhibits multiple work functions. This variation in work function across the upper electrode contact surface 103 can have a detrimental effect on devices utilizing such electrode structures 100, particularly resistance variable devices. For example, when used in conjunction with an array of PCRAM devices described above, upper electrode contact surfaces 103 exhibiting multiple work functions can cause a variation in the threshold switching voltage of each PCRAM memory device, causing each device to switch from a high resistance state to a low resistance state at a different voltage. Such variation in threshold switching voltage makes writing and erasing a given memory device in the array unreliable. Thus, it would be desirable to form an electrode structure which minimizes electrode workfunction variation.